Method of protecting power amplifiers

ABSTRACT

In a power amplifier, techniques are provided for protecting the power amplifier from temperature extremes resulting from conditions such as load mismatches. After sensing the temperature of the power amplifier, the operation of the RF power amplifier is controlled accordingly. In one example, the power amplifier is shut down when the sensed temperature reaches a threshold value, which can be set to a value just below the point where permanent damage would occur. In another example, when a threshold temperature is reached, the operation of the power amplifier is changed, to prevent the temperature of the power amplifier from rising to a level that would damage the power amplifier.

FIELD OF THE INVENTION

This invention relates to the field of power amplifiers. In particular,this invention is drawn to techniques for protecting power amplifiersfrom damage, such as damage from high temperatures.

BACKGROUND OF THE INVENTION

Power amplifiers are susceptible to damage when subjected to extremeconditions such as high temperatures. High temperatures can have variouscauses, including extreme current due to mismatched loads at theantenna, high duty cycle from data transmission, battery regulation whenrecharging a battery, or high ambient temperatures.

A typical RF power amplifier can be specified to handle load mismatcheson the order of a 10:1 ratio. A mismatch of 10:1 implies that a poweramplifier must drive loads of 5 ohms to 500 ohms. Nominally, the RFamplifier is specified is specified to drive a 50 ohm load. At theextremes of these mismatched conditions, a power amplifier, regardlessof which process technology has been implemented, could experiencestresses that have the potential to cause permanent damage to the poweramplifier.

In a power amplifier implemented using a complementary metal oxidesemiconductor (CMOS), when the load impedance at the antenna is high(e.g., 500 ohms), the node voltages could exceed the CMOS breakdownvoltage and cause permanent damage. When the load impedance of a load islow (e.g., 5 ohms), the current supplied to the load may be high enoughto result in thermal problems. This invention addresses the problemsassociated with thermal issues.

Some prior art power amplifiers use current sensors to prevent the poweramplifiers from supplying too much current to a load. In one example,current is sensed external to the power amplifier, and the poweramplifier is turned off when a current limit is exceeded. One problemwith this approach is that using only current sensing may inhibitcertain modes of operation which are acceptable, such as a high currentfor a short period of time. Also, some current sensing techniques do nottake into account environmental conditions such as ambient temperature,ventilation, heat sinking, etc. Another problem with this approachrelates to the fact that the system only knows the current drawn, whichcan result in flaws. For example, such a system can not take intoaccount the efficiency of the power amplifier or the true junctiontemperature of the die. Such a system is also inefficient, since theresistor will dissipate power, thus reducing overall efficiency.

SUMMARY OF THE INVENTION

A power amplifier according to one illustrative embodiment of theinvention uses one or more techniques for protecting the power amplifierfrom damage from high temperatures. In one embodiment, a control circuitchanges the operation of the power amplifier depending on one or moretemperature thresholds being reached. In one example, the controlcircuit shuts down the power amplifier in response to the sensedtemperature. In another example, the control circuit attempts to reducethe temperature of the power amplifier without shutting down the poweramplifier. These two examples can be operated within the same design.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitedin the figures of the accompanying drawings, in which like referencesindicate similar elements and in which:

FIG. 1 is a block diagram showing a power amplifier used in a wirelessapplication.

FIG. 2 is a flowchart illustrating one example of the operation of thepresent invention.

FIGS. 3-5 are timing diagrams illustrating examples of the operation ofthe invention.

FIG. 6 shows a block diagram of a circuit for sensing the temperature ofa power amplifier and generating the control signals.

FIG. 7 is a schematic diagram illustrating one example of a bandgapcircuit for generating a temperature independent reference signal.

FIG. 8 is a diagram of a power amplifier formed on an integratedcircuit.

DETAILED DESCRIPTION

In order to provide a context for understanding this description, thefollowing description illustrates one example of a typical applicationof the present invention. A power amplifier of the present invention maybe used for any desired applications, including wireless transmissionsystems such as mobile or cellular communication devices or otherwireless devices. In a wireless device, the wireless device may includea transceiver, an antenna module, and an antenna. Coupled between thetransceiver and the antenna module is an RF power amplifier foramplifying signals for transmission via the antenna. In the case of awireless mobile application, the invention may be applied to GSM, CDMA,PCS, DCS, etc., or any other wireless systems. This is just one exampleof an application of a power amplifier utilizing the present invention.The invention may also be used in any other application requiring apower amplifier.

Generally, the present invention provides techniques for protectingpower amplifiers from temperature extremes resulting from conditionssuch as load mismatches. In one example, an RF power amplifier isimplemented in an integrated circuit. The temperature of the integratedcircuit is sensed, and the operation of the RF power amplifier iscontrolled accordingly. In one example, the power amplifier is shut downwhen the sensed temperature reaches a threshold value, which can be setto a value just below the point where permanent damage to the integratedcircuit would occur. In another example, when a threshold temperature isreached, the operation of the power amplifier is changed, to prevent thetemperature of the power amplifier from rising to a level that woulddamage the integrated circuit. Examples of actions that can be takeninclude decreasing the output power level of the power amplifier,decreasing the duty cycle of the output of the power amplifier, droppinga call, instructing a user to take corrective measures, etc.

FIG. 1 is a block diagram showing a power amplifier 110 used in awireless application. The power amplifier 110 receives an RF inputsignal (RF_(IN)) and provides an amplified RF output signal (RF_(OUT))to an antenna 112 for transmission. A controller 114 is coupled to thepower amplifier 110 to control the operation of the power amplifier 110.In one example, the controller 114 is an external baseband controller,which controls the operation of the power amplifier 110. In anotherexample, the controller 114 is formed on the same integrated circuit asthe power amplifier 110.

As mentioned above, the present invention protects the power amplifierby sensing the temperature of the power amplifier and taking anappropriate action. In one example, two temperature thresholds areestablished. A first temperature threshold is set as a warning at apoint where a thermal fault has not occurred, but where a fault willoccur if the temperature continues to rise. For example, the firsttemperature threshold can be set at a point determined by how fast thepower amplifier can be cooled by changing certain conditions in thepower amplifier, without having to be shut down. If conditions in thepower amplifier can be changed such that the temperature will notcontinue to rise, then the power amplifier can be protected, withoutshutting down the power amplifier. A second threshold temperature can beset at a point where a thermal fault has occurred and the poweramplifier is shut down. For example, the second threshold temperaturecan be set just below the temperature where damage would occur. As aresult, the power amplifier will shut down before any damage can occur.

FIG. 2 is a flowchart illustrating one example of the operation of thepresent invention. The process shown in FIG. 2 begins with step 2-10,where the temperature of the power amplifier is sensed. At step 2-12,the process determines whether the sensed temperature exceeds thethreshold temperature T2. In one example, the threshold temperature T2is established at a point where thermal damage is about to occur if thepower amplifier continues to operate. If the sensed temperature exceedstemperature T2, the process proceeds to step 2-14 where the poweramplifier is shut down. After the power amplifier is shut down (step2-14), the temperature is still monitored. At step 2-16, the temperatureis sensed. At step 2-18, the process determines whether the sensedtemperature exceeds the threshold temperature T3. In one example thethreshold temperature T3 corresponds to a temperature where it would besafe to restart the power amplifier. At step 2-18, the processdetermines whether the sensed temperature still exceeds the thresholdtemperature T3. If so, then the power amplifier remains shut down, andthe temperature is sensed again. When the sensed temperature drops belowthreshold temperature T3, the process proceeds to step 2-20, where someaction is taken. In one example, the power amplifier is enabled. Otherexamples of actions are also possible. If, at step 2-12, the sensedtemperature does not exceed temperature T2, the process proceeds to step2-22. At step 2-22, the process determines whether the sensedtemperature exceeds the threshold temperature T1, which, in thisexample, is less than threshold temperature T2. In one example, thethreshold temperature T1 is established at a point where a fault isabout to occur, but where the temperature can be controlled by takingsome action. If the sensed temperature exceeds temperature T1, theprocess proceeds to step 2-24 where action is taken to attempt toprevent the temperature from reaching the temperature threshold T2.Examples of actions (or any combination of actions) that can be takenare described in more detail below. After action is taken, the processgoes back to step 2-10, where the temperature is sensed again. If it isdetermined that the sensed temperature does not exceed temperature T1,the process goes back to step 2-10, where the temperature is sensedagain. Note that the determinations outlined in steps 2-12 and 2-22could occur in any order, or simultaneously. Also, the present inventionmay only use one temperature threshold, and take action, or shut downthe power amplifier accordingly. Similarly, more temperature thresholdscan be used.

FIGS. 3 and 4 are timing diagrams illustrating examples of the operationof the invention. In the example of FIG. 3, temperature threshold T1 isreached, but temperature threshold T2 is not reached. The first signalshown in FIG. 3 is the external power amplifier enable signal (PAEN),which is high when the power amplifier is being used to amplify theinput signal RF_(IN). In FIG. 3, the PAEN is shown going high at timet₀. The next signal shown in FIG. 3 is a temperature signal 120, whichhas a voltage derived from the sensed temperature. As described above,the temperature of the power amplifier is sensed, and compared tothreshold temperatures T1 and T2. As shown, the temperature signal 120reaches threshold temperature T1 at time t₁. As a result, a thermallimit signal (TLIMIT) 122 goes high, and one or more actions are takenin an attempt to prevent the temperature from reaching temperaturethreshold T2. In this example, temperature T2 is not reached. At timet₂, the sensed temperature drops below temperature T1, signal 122 goeslow, and the power amplifier returns to a normal operating mode.

In the example of FIG. 4, temperature thresholds T1 and T2 are bothexceeded. In this example, the temperature threshold T1 is reached attime t₁ and signal 122 goes high. At time t₂, the sensed temperaturereaches temperature threshold T2, and shutdown signal (SHUTD) 124 goeshigh. When signal 124 goes high, the power amplifier is shut down toprevent the power amplifier from being damaged. As a result, thetemperature of the power amplifier decreases. In one example, the poweramplifier is reactivated when the temperature drops below thresholdtemperature T3 at time t₅. Signals 122 and 124 can be disabled atdifferent times depending on the application requirements.

FIG. 5 is another example of a timing diagram illustrating examples ofthe operation of the invention. In the example of FIG. 5, the poweramplifier enable signal (PAEN) is shown through three bursts. FIG. 5also shows an analog power control signal (APC), which relates to adesired power amplifier output level, and the sensed power amplifieroutput level (P_(OUT)) is also shown. In the example shown in FIG. 5,during the first burst, the temperature signal 120 reaches temperatureT1 at time t₁. When the temperature signal 120 reaches temperature T1,the thermal limit signal 122 goes high. In this example, the signal 122stays high until the beginning of the second burst (after a delay 123).At time t₂, the temperature signal 120 reaches threshold temperature T1again. Like during the first burst, the thermal limit signal 122 goeshigh until the beginning of the next burst. As shown, the temperaturesignal 120 continues to rise, and exceeds threshold temperature T2 attime t₃. When threshold temperature T2 is reached, the shut down signal124 goes high, causing the power amplifier to shut down. In thisexample, the signal 124 stays high until the beginning of the next burst(after a delay 125). As shown, the external PAEN signal and the externalAPC signal remain high during the burst, but the power amplifier outputsignal POUT drops after a shutdown delay 127.

The present invention may be implemented in any desired way. FIG. 6shows one example of a block diagram of a circuit sensing thetemperature of a power amplifier and generating the control signals 122and 124 shown in FIGS. 3 and 4. In this example, the temperature of thepower amplifier is sensed, and a temperature signal is compared to thethreshold temperatures. FIG. 6 shows a temperature sensor 130, which maybe comprised of any desired temperature sensing circuit. The temperaturesensor 130 generates a temperature signal 120, which is provided tofirst inputs of first and second comparators 132 and 134. The secondinput of the comparator 132 is coupled to reference signal T1 _(REF),which corresponds to the first temperature threshold T1. The secondinput of the comparator 134 is coupled to reference signal T2 _(REF),which corresponds to the second temperature threshold T2. In thisexample, reference signals T1 _(REF) and T2 _(REF) are temperatureindependent reference signals. An example of the generation of referencesignals T1 _(REF) and T2 _(REF) are described below, with respect toFIG. 7. When the sensed temperature exceeds the first temperaturethreshold T1, the output of the comparator 132 will go high (signal122). Similarly, when the sensed temperature exceeds the secondtemperature threshold T2, the output of the comparator 134 will go high(signal 124).

In one example, the temperature reference signals T1 _(REF) and T2_(REF) are temperature independent signals having voltages that do notvary significantly with changes in temperature. FIG. 7 is a schematicdiagram illustrating one example of a bandgap reference circuit 140 forgenerating the temperature reference signals. The circuit 140 includesswitching devices M1, M2, M3, Q1, and Q2 coupled as shown. In addition,op-amp 142 and resistors R1, R2, and R3 are coupled to the switchingdevices as shown. In the example shown in FIG. 7, a reference voltage isderived from the bandgap of silicon. In this example, the referencesignal is generated by summing voltages withproportional-to-absolute-temperature (PTAT) andcomplementary-to-absolute-temperature (CTAT) temperature coefficients,such that the resulting voltage T_(REF) is constant with changingtemperature. In the example shown in FIG. 7, the reference signalT_(REF) can be represented by the following equation:$T_{REF} + V_{BE2} + {\frac{R2}{R1}V_{T}{{\ln\left( \frac{A1}{A2} \right)}.}}$T_(REF) is then buffered by op-amp 144 and switching device M4 to set aprecision, temperature-independent current in resistor chain R_(SET).The resistor chain R_(SET) includes resistors R4, R5, and R6, which forma voltage divider to set reference signals T1 _(REF) T2 _(REF) todesired values, which depend on the sizes of the resistors. Note thatthe reference signals T1 _(REF) and T2 _(REF) can be generated in otherways as well. In another example, one or more reference signals areprovided from an external source (e.g., a specialized reference signalfrom the controller, a reference derived from the received PAEN (orother) signal, etc.). V3 is a voltage with a PTAT temperaturecoeffecient, and in this example, may be used for temperature sensor 130of FIG. 6.

In one embodiment, a power amplifier of the present invention is formedon an integrated circuit and is controlled by an external controller,such as baseband controller. FIG. 8 is a diagram of a power amplifier110 formed on an integrated circuit 150. FIG. 8 shows four interfacepins PAEN, SHUTD, APC, and TLIMIT. Note that the integrated circuit 150has additional pins, but they are not shown in FIG. 8. A first pin islabeled PAEN is adapted to receive the power amplifier enable signalfrom the controller. A second pin is labeled APC and is adapted toreceive an analog power control signal. The APC signal controls theoutput power of the power amplifier. During operation of the poweramplifier, the pins labeled SHUTD and TLIMIT provide data to thecontroller, as described above with respect to FIGS. 3 and 4. In anotherembodiment, information can be sent to the controller via a digitalinterface (e.g., a serial interface, etc.) or other interface. In theexample of a digital interface, the information sent to the controllercould also include the measured temperature, and the controller couldperform the comparisons and take the appropriate actions. The integratedcircuit can be formed using any desired manufacturing technology, suchas CMOS technology.

The controller can be designed to use the information from the pinsSHUTD and TLIMIT in any desired manner. In one example, when SHUTD goeshigh, the power amplifier turns itself off, so the SHUTD signal that thecontroller receives simply lets the controller know that the poweramplifier has been shut down. In another example, the power amplifierdoes not shut itself down, and relies on the controller to shut down thepower amplifier (e.g., by lowering the PAEN signal or the APC signal)after receiving the SHUTD signal from the power amplifier. Thecontroller may use information from the TLIMIT signal in many ways. Forexample, the controller can take a number of actions that should help tolower the temperature of the power amplifier. Examples of actionsinclude reducing the output power, reducing the duty cycle, shuttingdown the power amplifier, dropping the call, renegotiating communicationparameters with a base station, etc. In another example, the controllercan convey a message to a user (e.g., an audible message, visualmessage, etc. instructing a user to take corrective action) when thecontroller believes a problem can be addressed by the user. For example,when it is possible that a load mismatch is caused by a large metallicobject positioned near the antenna, the controller could instruct theuser to move away from the object.

In the preceding detailed description, the invention is described withreference to specific exemplary embodiments thereof. Variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the invention as set forth in the claims.The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A method of protecting an RF power amplifier comprising: sensing thetemperature in an RF power amplifier; and changing the operation of theRF power amplifier when the sensed temperature reaches a thresholdvalue.
 2. The method of claim 1, further comprising shutting down the RFpower amplifier in response to the threshold value being reached.
 3. Themethod of claim 1, further comprising decreasing the output power of theRF power amplifier in response to the threshold value being reached. 4.The method of claim 1, further comprising decreasing the duty cycle ofthe output of the RF power amplifier in response to the threshold valuebeing reached.
 5. The method of claim 1, wherein the RF power amplifieris used in a mobile communication system, the method further comprisingdropping a call in response to the threshold value being reached.
 6. Themethod of claim 1, further comprising sending a message to a user of theRF power amplifier in response to the threshold value being reached. 7.The method of claim 6, wherein the message instructs the user to take anaction.
 8. The method of claim 7, wherein the message instructs the userto move.
 9. The method of claim 1, wherein sensing the temperature inthe RF power amplifier includes comparing a temperature dependent signalwith a reference signal to determine when the sensed temperature reachesthe threshold value.
 10. The method of claim 9, wherein the referencesignal is a temperature independent voltage signal.
 11. The method ofclaim 9, wherein the reference signal is a bandgap reference signal. 12.The method of claim 1, wherein changing the operation of the RF poweramplifier when the sensed temperature reaches a threshold value includesan action that attempts to reduce the temperature of the RF poweramplifier.
 13. The method of claim 12, further comprising shutting downthe power amplifier when the sensed temperature reaches a secondthreshold value.
 14. A method of protecting an RF power amplifiercomprising: establishing a first temperature threshold value;establishing a second temperature threshold value; sensing thetemperature in the RF power amplifier; changing the operation of the RFpower amplifier when the sensed temperature reaches the firsttemperature threshold value; and shutting down the RF power amplifierwhen the sensed temperature reaches the second temperature thresholdvalue.
 15. The method of claim 14, wherein the RF power amplifiergenerates an RF output signal, the method further comprising decreasingthe power of the RF output signal in response to the first temperaturethreshold being reached.
 16. The method of claim 14, wherein the RFpower amplifier generates an RF output signal, further comprisingdecreasing the duty cycle of the RF output signal in response to thefirst temperature threshold being reached.
 17. The method of claim 14,wherein the RF power amplifier is used in a mobile communication system,the method further comprising dropping a call in response to the firsttemperature threshold being reached.
 18. The method of claim 14, furthercomprising sending a message to a user of the RF power amplifier inresponse to the first temperature threshold being reached.
 19. Themethod of claim 18, wherein the message instructs the user to take anaction.
 20. The method of claim 19, wherein the message instructs theuser to move.
 21. The method of claim 14, further comprising determiningwhen the first temperature threshold value is reached by comparing atemperature dependent signal with a first reference signal.
 22. Themethod of claim 14, further comprising determining when the secondtemperature threshold value is reached by comparing a temperaturedependent signal with a second reference signal.
 23. The method of claim22, wherein the first and second reference signals are temperatureindependent signals.
 24. The method of claim 23, further comprisingusing a bandgap reference circuit to derive the first reference signal.25. The method of claim 24, further comprising using a bandgap referencecircuit to derive the second reference signal.